What Can Squid Hear? And Why Does It Matter?

Ears are amazing. They help you keep balance, turn a newborn's face towards its mother's voice, and alert us to escape danger. Increasingly, these wondrous organs are the target of new frontiers in environmental protection. Human irritation from jet engines, lawn mowers, and wind turbines, much less SunChips' loud, crinkly bioplastic bags, stands as a reminder of the importance of the aural environment. Research seeking to understand the impacts of noise pollution in the marine environment document threats to whales and fish. But researchers studying squid hearing hope to learn even more. Can studying squid hearing help us understand how all ears work?Squid anchor many oceanic foodchains. These delicacies, ebbing and flowing with the currents, serve as a smorgasbord for denizens of the sea. Biologist T. Aran Mooney, of the Woods Hole Oceanographic Institution (WHOI), asks the questions: "Can squid hear? Is their hearing sensitive enough to hear approaching predators? How do squid and other marine species rely on sound to interact, migrate, and communicate?"

Targeting a seminal species in the food web commonly serves science by predicting potentially wide-ranging impacts on the entire ecological niche which depends upon that species for survival. Learning about how marine noise pollution disrupts squid behavior can be justified on this ground alone. But Mooney may be on the trail of much broader questions: How do ears work? How did ears evolve?

Do Squid Have Ears?The reason Mooney may be on to something bigger lies in the answer to the question: do squid have ears? Squid have very primitive organs, called statocysts, at the base of their brains. Although the name sounds like something you might want a surgeon to look into immediately, these organs serve a key purpose. The statocysts contains a tiny grain of calcium nested in a fluid-filled sac lined by hairs -- not dissimilar to a human cochlea.

When a squid moves, the calcium grain, scientifically named a statolith, presses against the hairs lining the statocysts. This helps the squid to sense its motion in the water, so it can steer and keep its balance. But do these cochlean sacs serve for hearing as well as balance? Are we looking at the primitive foundations of the evolution of ears?

To find out, Mooney tests the nerves leading from the statocysts to the squids' brains. Comparing this with background signals from non-hearing parts of the squids' bodies, Mooney can determine whether the statocysts are reacting to noises in the squids' environment. It is a technique similar to that used to check for hearing in human infants.

What Mooney Learned About Squid EarsMooney's research indicates that squid do hear. But they detect noises only up to about 500 Hz. Compare that to human hearing, which detects sounds ranging from 20 Hz to 20,000 Hz. More importantly, this implies that squid do not hear the higher frequency sonar impulses used by dolphins and whales for echolocating their prey. The squid probably first "hear" their predators when the compression wave pushed ahead of the swimming creatures strike them -- too late to maneuver away. Which leaves the squid as sitting ducks on the saline smorgasbord.

But this also begs the question: did the evolution of squid ears stall after attaining motor control because hearing fails to benefit squid by helping them avoid their predators? Indeed, the WHOI team made another discovery. By putting squid into a CT scanner, Mooney found that they are invisible! The squid are nearly the same density as the water around them, making them difficult for echolocating predators to detect.

Mooney's research into squid hearing will continue. In particular, Mooney wants to understand whether anthropogenic noise -- there are plenty of sounds of human origin within the squids' frequency spectrum -- affects the squid, and if squids can localize the direction of the sounds they can hear. Localization is another important aspect of hearing, as it helps us respond to the intentional communication of those around us, as well as know in which direction to flee from danger.

This fascinating area of research will certainly help us better manage human noise pollution threats in the marine environment. If we learn how human ears evolved and more about how they work along the way, that will be worth making some noise over.